Security vendor: Firewalls defenceless against new attack

Is 'AET' packet probing behind major attacks?

Why are apparently well-defended networks belonging to the government, military and monied corporates falling to large-scale targeted attacks so regularly?

According to Finnish security vendor Stonesoft, part of the answer could lie with what the company terms 'Advanced Evasion Techniques' (AETs), jargon for an obscure class of packet-based probing at the ground level of the TCP/IP stack that firewalls are designed to stop.

The trouble, claims Stonesoft, is that for a clutch of 31 obscure AETs nobody has paid much attention to until recently, the protection level turns out to be close to zero.

It's a contentious claim and the evidence is hazy, but does it stand up? Just about, although the hard-to-assess bit is whether such attacks have actually been used to do any real damage.

The role of AETs is to probe for vulnerable servers and other systems in an industrial and automated way without being detected by security systems. In Stonesoft's analysis, the range of AETs being employed against networks is far greater than previously believed, and they are being combined in complex, multiprotocol probes that firewalls just can't see.

If accurate, this sounds significant. It means that every firewall in the world is unable to detect probes used to hunt down servers vulnerable to the application-layer exploits that fuelled major cyber-infrastructure incidents of recent times such as Stuxnet and this year's Aurora attacks on Google and others.

The company arrived at its conclusion, it said, after trying to fix problems in its own security appliances during certification programs, during which it built its own testing tools.

"The dynamic and undetectable nature of these advanced evasion techniques has the potential to directly affect the network security landscape," said Stonesoft COO, Juha Kivikoski. "The industry is facing a non-stop race against this type of advanced threats and we believe only dynamic solutions can address this vulnerability."

It has no direct evidence that high-profile attacks have been preceded by AET probing but suspects that given the vulnerability of firewalls they have uncovered, they might turn out to be a missing 'missing link' in an unknown number of incidents.

Stonesoft reported its findings to CERT-FI in Finland some time ago and has had the results verified by leading certification organisation, ICSA Labs, which lends some support to its claims.

A contentious issue is what can be done to patch the problem in security appliances, which covers intrusion prevention systems, firewalls, and even major classes of router.

Stonesoft pins some of the blame on the ASIC-based design of modern appliances, which is supposed to improve performance but at the cost of baking in static security rules and defences into hardware. Conveniently, its own design is based on virtual appliances driven by software, which would be easier to update or 'patch' dynamically to take account of emerging AETs.

This key question is whether AETs are important to modern cybercrime and it seems plausible that in some form they might be. It has long been assumed that advanced probing and bypassing techniques were used by hackers who, understandably, failed to document the success of such techniques. What Stonesoft has possibly uncovered found the assumption that they couldn't or wouldn't be used in sophisticated combinations is probably out of date at the very least.

Intrusion prevention systems (IPS) have a spotty record at best for blocking carefully-crafted attacks and some have even questioned whether they are worth bothering with at all.

The company has published more detail on AETs on a special website. Prepare for rival firewall and security companies to rubbish its claims as marketing spin.

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